Zirconium based metallocene polymerization catalysts, such as, bis(cyclopentadienyl)zirconium dichloride, are well known and are commonly used as ethylene polymerization catalysts when combined with activators, such as, for example, methylaluminoxane (MAO). A description of such catalysts can be found, for example, in Angew. Chem. 88, 689, 1976, Justus Liebigs Ann. Chem. 1975, 463, and U.S. Pat. No. 5,324,800, herein incorporated by reference. Zirconium based metallocenes can be quite active, but unfortunately, these metallocenes also produce a fairly narrow molecular weight distribution.
For many extrusion grade applications, such as film, pipe, and blow molding, polymers having broad molecular weight distributions are preferred. Especially preferred are so-called “bimodal distribution” polymers because of the superior toughness imparted to the final manufactured resin part. See, for example, U.S. Pat. Nos. 5,306,775 and 5,319,029, herein incorporated by reference. The superior toughness can result from concentrating the short chain branching in the high molecular weight portion of the molecular weight distribution. Extremely long and highly branched chains can be more effective as tie molecules between the crystalline phases. These tie molecules can impart higher impact resistance and environmental stress crack resistance to bimodal polymers.
To produce such bimodal polymers from metallocene catalysts, it is necessary to combine two metallocenes. A first metallocene is utilized to produce a low molecular weight polymer having little branching. Zirconium based metallocenes can function well in such a role. A second metallocene is utilized to produce the high molecular weight polymer, and this second metallocene should also simultaneously incorporate comonomers, such as hexene, very well. In this way, the longest chains contain the most branching, which is ideal for the production of bimodal polymers.
Unfortunately, the requirements of the second metallocene has been difficult to fill. Of the zirconium based metallocenes described previously, few generate very high molecular weight polymer. Of these few, activity or stability is often poor, and comonomer incorporation is not impressive. A second class of metallocene catalysts, called half-sandwich titanium based metallocenes, do produce very high molecular weight polymer, and some even incorporate hexene well. See Organometallics, 1966, 15, 693-703 and Macromolecules 1998, 31, 7588-7597. Half-sandwich titanium based metallocenes have a titanium bonded to one cyclopentadienyl, indenyl, or fluorenyl group. However, these compounds are not noted for their high activity.
There is a need in the polymer industry for a metallocene catalyst or organometal catalyst that produces high molecular weight polymer, has a high activity, and incorporates comonomers efficiently that can be used alone or in combination with other metallocenes.
It is an object of this invention to provide a first organometal compound capable of producing high molecular weight polymers.
It is another object of this invention to provide a process for producing a first catalyst composition. The process comprises contacting at least one first organometal compound and at least one activator.
It is another object of this invention to provide the first catalyst composition.
It is another object of this invention to provide a polymerization process. The process comprises contacting the first catalyst composition with one or more alpha olefins in a polymerization zone under polymerization conditions to produce a high molecular weight polymer.
It is another object of this invention to provide the high molecular weight polymer.
It is another object of this invention to provide a process for producing a second catalyst composition capable of producing bimodal polymers. The process comprises contacting the first organometal compound, at least one activator, and at least one second organometal compound.
It is another object of this invention to provide the second catalyst composition capable of producing bimodal polymers.
It is a further object of this invention to provide a process for the production of bimodal polymers. The process comprises contacting the second catalyst composition with one or more alpha olefins in a polymerization zone under polymerization conditions to produce the bimodal polymers.
It is yet a further object of this invention to provide the bimodal polymer.